There is an immediate and growing need for satellite-based, global communications on-demand, between a hand-held user transceiver and a central gateway or hub. This need, which applies to both the U.S. government and non-government sectors, includes 24 hour-a-day random access communications support for emergency indication, small sensors, law enforcement, and many other "remote" user scenarios. The emphasis is on the ability of the satellite system to accommodate transmissions from hand-held units at anytime and anywhere; communications back to the user must also be supportable, but typically in reaction to a transmission from the user. Such hand-held, random-access communication scenarios cannot be accommodated by existing commercial satellite communication systems, but, according to this invention, can be readily supported by a novel and unique utilization of NASA's Tracking and Data Relay Satellite System (TDRSS) or a similar satellite system. The associated TDRSS support, coupled with the transceiver technology and implementation, are the subjects of this invention.
Most communication satellites operate at geosynchronous altitude, an altitude of about 22,000 miles, at which point the earth's disk appears approximately 20 degrees across. These satellite communication systems have traditionally utilized broad-coverage antennas to concurrently receive signals from, and transmit signals to, regional or near-hemisphere areas, while remaining over a fixed spot on the earth's equator. The broad antenna beam, at typical frequencies (e.g., microwave), corresponds to a small-area transmit-receive antenna. This, in turn, limits the electromagnetic power the antenna can intercept. The result is that, for acceptable communication quality, users on the ground.sup.1 must have relatively large antennas and/or transmit many watts of power; this, in turn, typically leads to transceivers that cannot be hand-held and, further, precludes efficient battery-powered operation. FNT .sup.1 "Ground" is used generically and refers equally to land, sea and air users.
Typical satellite transponders (that is, the on-board equipment for relaying signals within a given frequency bandwidth) are in essence amplifier-frequency-shifters which can accept signals from any user-transmitter on the ground operating within the band covered, amplify those signals, shift their frequency and retransmit them through another antenna to a central gateway. Since the signals are not demodulated or signal-processed on-board the satellite, there is no processing gain to compensate for low signal power.
Special purpose communication satellites (e.g., for the Department of Defense) have been built for a variety of purposes. With a larger antenna on the satellite, it is possible to communicate with a user on the ground having a correspondingly smaller antenna and/or transmitter power. In this case, however, the beamwidth of the satellite's antenna is reduced, thereby requiring the location of the ground user to be known, and the satellite's antenna tracked to that location. Were the antenna mechanically tracked by rotating itself or the entire satellite, that would use propellant at an unacceptable rate; what's more, it could serve concurrently only users in a small area of the earth. Electronic antenna steering provides a highly attractive alternative that eliminates the disadvantages of mechanical steering while simultaneously providing the ability to focus on (or null) many regions concurrently with high (or low, for nulling) gain; electronic steering can also be accomplished much more rapidly than mechanical steering, again without any incurred mechanical satellite motion. Electronic steering is more expensive than traditional non-steerable antennas, and have heretofore appeared mainly on military satellites. Furthermore, even on such military satellites, the number of simultaneous receive beams that can be formed, and their operational flexibility, has been limited by the specific on-board beamforming capability employed. In this regard, the electronic beamforming capability used by the TDRSS is especially unique.
To satisfy its needs for global communications with low earth-orbiting spacecraft, NASA has developed the Tracking and Data Relay Satellite Systems (TDRSS), which includes geosynchronous satellites that are able to electronically steer an on-board phased-array antenna. This phased array views the entire earth's disk, but can form many simultaneous beams to support reception of many independent user transmissions; each such beam has a beamwidth considerably narrower than the earth's disk and thus also provides considerably higher gain than an earth coverage beam. Furthermore, this same phased-array can form a single narrow beam at a time to provide high power transmissions back to the user; this beam can be independent of, or directly related to, any of the many simultaneous receive beams. As such, both the receive (inbound or return link) and transmit (outbound or forward link) beams are sufficiently powerful to accommodate low-power; hand-held user transceivers; not only is this operationally attractive to the user but it also provides the added benefit of extended battery lifetime and reduced exposure to RF emissions.
Electronic beam steering requires that signals from a number of separate antenna elements, most commonly arranged in a planar area, be phase-shifted by amounts depending on the distance of the element from the center of the array and the direction in which the beam is to form. Whether the application is radar or communications, such antennas typically have their beamforming accomplished at the antenna. In this regard the TDRSS is unique, in that the inbound (i.e., return link) beamforming is performed on the ground. Specifically, the TDRSS transmits the signal, received by each on-board antenna element, separately to the ground station in a composite, frequency-multiplexed signal. Since the coverage of each element of the TDRSS is more than the angle of the earth's disk, the combination of signals sent to the ground can be combined on the ground to "form" a much narrower beam and to direct it, free of any mechanical inertia. This has several advantages relative to conventional approaches of beamforming at the antenna. First, a beamformer on the ground can be replaced if a failure occurs. Second, the number of independent beamformers can be much greater on the ground than can be possibly placed on-board a satellite. Third, the number of independent beamformers can be expanded and independently allocated to independent users, if needed, after the satellite is in orbit, and one or more receivers can be attached to each beamformer. Finally, the beamforming algorithms can evolve and improve with technology, if the beamforming is accomplished on the ground and can include split beams, nulling, and other forms of enhanced antenna pointing. Clearly, all of these advantages of ground-based beamforming yield a greatly increased satellite "return on investment".
Many global voice and data communication needs, both government and non-government, remain unmet through application of conventional communication satellites. More particularly, few (if any) satellite sensor or communication systems can communicate flexibly (e.g., demand or random access) and successfully with small, low-powered, hand-held, low-data-rate ground-based user transceivers, or with remote instruments or controllers not equipped with large antennas or, equivalently, with high output power.
In addition to its services using electronically steered antennae, TDRSS also provides additional services via mechanically steered antennae. These single access (SA) services, at S, Ku and future Ka band, provide links with attractive bandwidth and link closure properties. Use of these services require scheduling; however, normal mission users do not require all of the schedulable service and spare satellites on orbit have no scheduled use. As such, there is a strong potential to use excess capacity within the SA services to also accommodate new ground users. For example, field users and/or users on the ground terminal side could store information over, for example, a period of hours. At a scheduled time, when no normal users required services, all or part of the stored information could be forwarded (i.e. transmitted) through TDRSS via one of the available services. Hence, this store-and-forward technique could be used to pass data to/from the field during intervals for which TDRSS had no other service requirement. As another example, since each TDRSS single access antennae is multi-band (S, Ku now; S, Ku, Ka in the future), only one frequency is typically used at a time. As such, if an independent user is within the beamwidth of an already scheduled single-access service, the independent user can take advantage of the "free" frequency to obtain single access service without the need for separately scheduled single access time.
Given the wide range of services and service types, there are numerous potential ground-based uses of TDRSS, even outside the area of hand-held transceivers, which are worthy of consideration. For example:
1. The ability to support high bandwidth broadcast and low rate request channels in a single satellite makes TDRSS a candidate for asymmetric broadcast services (e.g., the Global Broadcast System) for providing wireless Internet access, etc. PA1 2. Packet data transfer via TDRSS, with short (e.g. &lt;1 second) data packets, is possible using modified receiver technology to achieve rapid signal acquisition. Current TDRSS operations provide continuous service over contact intervals which may range from seconds to minutes in duration. PA1 3. Field equipment may be tailored to mesh with services. For instance, low/high gain antennae, wide/narrow beam width antennae, low/high power amplifier, etc. can be employed to satisfy specific ground applications. PA1 4. TDRSS is a "bent pipe" and data flow through it has no formal requirements in terms of content, security/encryption, coding, etc. In essence, all data may be passed over TDRSS given the RF signal characteristics are acceptable.
The object of this invention is to provide an improved global satellite communication system that uniquely applies the TDRSS without impact to its prime mission of supporting low-earth-orbiting science spacecraft; or another suitably implemented satellite system. This invention encompasses both the satellite system concept including the ground terminal, and the ground-based transceiver design and implementation required for successful system operation.